Matabele,

As long as the hoop strength of the barrels can contain the pressure within the cartridge, and that shouldn't be a problem with a 20ga action and modern barrels, everything should be good there. I'm pretty sure Ellis has this calc in his book.

I think people sometimes focus on cartridge pressure without taking actual bolt thrust into account as this is the force back into the action and probably a more important consideration in selecting a suitable shotgun action. You can have two different cartridges loaded to the same pressure, yet the round with the smaller case head diameter will have less bolt thrust. I did some rough calc's for the cartridges mentioned above to show this. As an aside, I don't have the knowledge or expertise to be giving advice on this subject, just giving you a guide in which direction to focus your enquiries. This info should be confirmed by someone with more knowledge than I.

Using SAAMI max pressure I get bolt thrust as follows:

* 30-30 Win = 5,277 lb
* 30-40 Krag = 6,909 ib
* 303 Brit = 7,249 lb

Once again, these figures should be confirmed by someone else. You really need to check the proof stamping on the shotgun to confirm the action will handle this thrust safely.

But.......this is only part of the equation. I've read some interesting testing which showed that the more taper a case has, the more thrust is transfered back into the action. This suggests going with an Ackley improved variant of the cartridge would be the way to go.

Hope this helps.

Rhodes
DRSS